I'm trying to pull tasks from an asyncio queue and call a given error handler if an exception occurred. Queued items are given as dictionaries (enqueued by enqueue_task) which contain the task, a possible error handler, and any args/kwargs the error handler may require. Since I'd like to handle any errors as the tasks complete, I map each task to the dequeued dictionary and attempt to access it if an exception occurs.
async def _check_tasks(self):
try:
while self._check_tasks_task or not self._check_task_queue.empty():
tasks = []
details = {}
try:
while len(tasks) < self._CHECK_TASKS_MAX_COUNT:
detail = self._check_task_queue.get_nowait()
task = detail['task']
tasks.append(task)
details[task] = detail
except asyncio.QueueEmpty:
pass
if tasks:
for task in asyncio.as_completed(tasks):
try:
await task
except Exception as e:
logger.exception('')
detail = details[task]
error_handler = detail.get('error_handler')
error_handler_args = detail.get('error_handler_args', [])
error_handler_kwargs = detail.get('error_handler_kwargs', {})
if error_handler:
logger.info('calling error handler')
if inspect.iscoroutinefunction(error_handler):
self.enqueue_task(
task=error_handler(
e,
*error_handler_args,
**error_handler_kwargs
)
)
else:
error_handler(e, *error_handler_args, **error_handler_kwargs)
else:
logger.exception(f'Exception encountered while handling task: {str(e)}')
else:
await asyncio.sleep(self._QUEUE_EMPTY_SLEEP_TIME)
except:
logger.exception('')
def enqueue_task(self, task, error_handler=None, error_handler_args=[],
error_handler_kwargs={}):
if not asyncio.isfuture(task):
task = asyncio.ensure_future(task)
self._app.gateway._check_task_queue.put_nowait({
'task': task,
'error_handler': error_handler,
'error_handler_args': error_handler_args,
'error_handler_kwargs': error_handler_kwargs,
})
However, when an exception occurs, it appears the task being used as a key is not found in the details dictionary, and I receive the following error:
KeyError: <generator object as_completed.<locals>._wait_for_one at 0x7fc2d1cea308>
Exception encountered while handling task: <generator object as_completed.<locals>._wait_for_one at 0x7fc2d1cea308>
Traceback (most recent call last):
File "/app/app/gateway/gateway.py", line 64, in _check_tasks
detail = details[task]
KeyError: <generator object as_completed.<locals>._wait_for_one at 0x7fc2d1cea308>
When task is yielded by asyncio.as_completed, it seems to be a generator
<generator object as_completed.<locals>._wait_for_one at 0x7fc2d1cea308>
when I expect it to be a task
<Task pending coro=<GatewayL1Component._save_tick_to_stream() running at /app/app/gateway/l1.py:320> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x7fc2d4380d98>()]>>
Why is task a generator instead of the original task after being yielded by asyncio.as_complete? Is there a way to access the original task?
Why is task a generator instead of the original task after being yielded by
asyncio.as_complete?
The problem is that as_completed is not an async iterator (which you'd exhaust with async for), but an ordinary iterator. Where an async iterator's __aiter__ can suspend while awaiting an async event, an ordinary iterator's __iter__ must immediately provide a result. It obviously cannot yield a completed task because no tasks have yet had time to complete, so it yields an awaitable object that actually waits for a task to complete. This is the object that looks like a generator.
As another consequence of the implementation, awaiting this task gives you the result of the original task rather than a reference to the task object - in contrast to the original concurrent.futures.as_completed. This makes asyncio.as_completed less intuitive and harder to use, and there is a bug report that argues that as_completed should be made usable as an async iterator as well, providing the correct semantics. (This can be done in a backward-compatible way.)
Is there a way to access the original task?
As a workaround, you can create an async version of as_completed by wrapping the original task into a coroutine that finishes when the task does, and has the task as its result:
async def as_completed_async(futures):
loop = asyncio.get_event_loop()
wrappers = []
for fut in futures:
assert isinstance(fut, asyncio.Future) # we need Future or Task
# Wrap the future in one that completes when the original does,
# and whose result is the original future object.
wrapper = loop.create_future()
fut.add_done_callback(wrapper.set_result)
wrappers.append(wrapper)
for next_completed in asyncio.as_completed(wrappers):
# awaiting next_completed will dereference the wrapper and get
# the original future (which we know has completed), so we can
# just yield that
yield await next_completed
That should allow you to get the original tasks - here is a simple test case:
async def main():
loop = asyncio.get_event_loop()
fut1 = loop.create_task(asyncio.sleep(.2))
fut1.t = .2
fut2 = loop.create_task(asyncio.sleep(.3))
fut2.t = .3
fut3 = loop.create_task(asyncio.sleep(.1))
fut3.t = .1
async for fut in as_completed_async([fut1, fut2, fut3]):
# using the `.t` attribute shows that we've got the original tasks
print('completed', fut.t)
asyncio.get_event_loop().run_until_complete(main())